This invention in accordance with one aspect thereof is concerned with drilling tools of the type which may be used for drilling oil and gas wells to a great depth in the earth's crust.
It is common practice to provide such a drilling tool in which the drill bit which is disposed at the lower end of the tool is operatively rotated, during which fluid commonly referred to as drilling mud is pumped through the drill string and the associated heavy drill collars to flushing ducting in the drill bit from which the drilling mud exits in the form of a pressurised jet or jets, this jet or jets of drilling mud serving to clean the bottom of the bore hole being cut by the drill bit with the cuttings and chips formed by the drill bit being carried upwardly by the drilling mud along an annular space between the drill string and the wall of the bore hole formed by the drill bit to the surface at which the drilling mud may be cleaned and reused.
As will be appreciated, the torque required to rotate the long drill string is very considerable in view of the weight of the drill collars disposed above the drill bit and, more particularly, in view of the frictional resistance which requires to be overcome, the drill string in view of its very considerable length seldom being truly straight so that this frictional resistance is thereby increased. With the view to eliminating or at least minimizing these disadvantages it has hitherto been known to provide a down hole motor for operatively driving the drill bit and which is disposed at the lower end portion of the drill string and above the drill bit, or to provide a hammer or percussive motor by which a hammer or percussive effect is operatively applied to the drill bit.
A number of designs of such down hole motor and percussive or hammer motor devices have been developed, one of the most widely used designs which utilizes a positive displacement motor being that marketed under the trademark Dynadrill, but certain disadvantages inherent to this design such as, for example, the relatively low torque available from the motor and its relatively short operating life together with the fact that a principal component of the motor is constructed from an elastomer which precludes use of the motor in thermal bore holes have restricted the use of this design of device to cases where higher costs and shorter operational life are acceptable.
In alternative designs of down hole motors the motors are constituted by axial flow turbines, but these forms of motors which are known as turbodrills require that at the surface the fluid for operation of the turbines be at an extremely high pressure and high volume fluid flows are required. Furthermore, such motors are also very expensive to manufacture and maintain, and since low torque but high rotational speed are characteristics of these turbodrills the use of such down hole motors which have mainly been restricted to cases where the drill bits are diamond tipped is not ideal where the motors operatively drive core rock bits.
In all the above-described prior forms of down hole motors the drill bits are operatively continuously rotated so that in these down hole motors, and also in percussive or hammer devices where the drill bits are operatively continuously rotated from the surface by the drill string, there are still substantial associated frictional forces and there is an inevitable reduction in the pressure of the drilling mud leaving from the drill bits because of the work required to be performed by the drilling mud in rotating the drill bits, or in actuating the percussive or hammer motors. The cleaning action of the jet or jets on the bottom of the bore hole being formed by the drill bit is thus correspondingly reduced. Furthermore, the continuous operative rotation of the drill bit impedes the drilling mud jet or jets from the drill bit so that turbulence is created with resultant loss in the pressure and energy of the drilling mud jet or jets and hence with a resultant reduction in the cleaning action on the bottom of the bore hole, this turbulence being further increased if the drill bit incorporates toothed cones which also rotate during operative rotation of the drill bit. Thus, cuttings and chips tend to remain on the bottom of the bore hole and act as a cushion against which the drill bit operates thereby significantly reducing the rate of drilling.
In the hitherto known forms of percussive or hammer devices the member which provides the percussive or hammer effect is spaced from the drill bit, or from a member on which the drill bit is directly or indirectly mounted, except at the moment of impact during each cycle of operation of the device, and in use such percussive or hammer devices can result in breakage of the drill bit or other damage to the device.
A further aspect of the present invention is concerned with axial thrust bearings which may be used in down hole drilling tools in accordance with said one aspect of the present invention but which may also be used in other forms of apparatus particularly where high axial thrust forces are involved, and a still further aspect of the present invention is concerned with the provision of a method of assembly of such an axial thrust bearing.